1. Field of the Invention
The present invention relates to a reproduction power test method for setting an optimal reproduction power when reproducing record information recorded on a magneto-optical medium by displacing a domain wall in the recording magnetic domain, and an optical information reproduction apparatus utilizing the method.
2. Related Background Art
Formerly, since the line recording density of an optical disk largely depends on the laser wavelength of a reproduction optical system and the numerical aperture of an objective lens and both a settled laser wavelength xcex of the reproduction optical system and a settled numerical aperture NA of the objective lens determines the radius of a beam waist, the detectable spatial frequency at the time of reproduction of a signal is limited to the order of 2NA/xcex. Thus, to implement a high density in a conventional optical disk, it is necessary to shorten the laser wavelength of a reproduction optical system and to increase the numerical aperture NA of an objective lens, but there is a limit to an improvement in laser wavelength and numerical aperture NA also. Accordingly, there has been developed a technique for improving the recording density by contriving the configuration of a recording medium and a reading method.
Also, the present applicant has proposed an information reproduction method capable of reproducing the signal recorded at a bit length of not greater than the diffraction limit of light in Japanese Patent Application Laid-Open No. 6-290496. The information reproduction method of the same publication will be briefly described referring to FIGS. 6A to 6C. Incidentally, here, the case of a 3-layered recording medium comprising a first magnetic layer (reproduction layer), a second magnetic layer (switching layer) and a third magnetic layer (recording layer) will be described as an example. The reproduction principle in the case of reproducing an information item on the basis of a temperature gradient formed by a reproducing light spot itself will be described. First, FIG. 6A is a sectional view of a recording medium. The magnetic layer of this medium takes a 3-layered structure comprising a first magnetic layer 601, a second magnetic layer 602 and a third magnetic layer 603 stacked in sequence. Arrowhead 604 in the respective layers represents the direction of atomic spins and a domain wall 605 is formed at the border between domains mutually opposite in direction of spins. Numeral 650 denotes a reproducing beam spot and Arrowhead 651 represents the moving direction of a recording medium. FIG. 6B shows a recording signal of the recording layer.
FIG. 6C is a graph showing the temperature distribution formed on a recording medium. This temperature distribution induced by a reproducing light spot itself on the medium. Here, at the position Xs1, the medium temperature becomes a temperature Ts near the Curie temperature of the second magnetic layer. In this case, a force displacing a domain wall in a decreasing direction of domain wall energy acts if there is a gradient in the density of domain wall energy. In the first magnetic layer 601, since the domain wall coercivity is small and the domain wall mobility is large, a domain wall is easily displaced singly by this force.
In a region prior to the position Xs1, since the medium temperature is lower than Ts and this region is in exchange coupling with the third magnetic layer 603 large in domain wall coercivity, a domain wall in the first magnetic layer 601 is also fixed at the position corresponding to that of the domain wall in the third magnetic layer 603. At this time, if the domain wall 606 is situated at the position Xs1 of the medium as shown in FIG. 6A, the medium temperature rises up to a temperature Ts near the Curie temperature of the second magnetic layer 602 and the exchange coupling between the first magnetic layer 601 and the third magnetic layer 603 is cut. The hatched portion of FIG. 6A corresponds to this.
As a result, the domain wall 606 in the first magnetic layer 601 is xe2x80x9cinstantaneouslyxe2x80x9d displaced to a region higher in temperature and smaller in domain wall energy density, i.e. a peak position of medium temperature as indicated by Arrowhead 607. Thereby, the size of the magnetic domain in the first magnetic layer 601 at the portion irradiated with a reproducing light spot is enlarged relative to that of the magnetic domain in the third magnetic layer 603. In this manner, even an infinitesimal record bit, incapable of being reproduced by an ordinary reproduction scheme, becomes a domain length capable of being reproduced on a magnetic layer under influence of the optical diffraction limit, so that a reproduction signal having much the same reproduction amplitude as with the reproduction of a record bit capable of being reproduced by an ordinary reproduction scheme and a steeper leading/training characteristic is obtained and the signal recorded at a bit length inferior to that of the diffraction limit of light. Incidentally, hereinafter, the medium displacing a magnetic domain wall in the recording magnetic domain to reproduce an information item is referred to as a domain-wall displacement type magneto optical medium.
Since such a displacement of a domain wall occurs depending upon the positional relation between the isothermal line of a temperature Ts near the Curie temperature of the second magnetic layer and the recording magnetic domain, however, the displacement timing of the magnetic domain depends upon the temperature on the surface of a substrate. Besides, the temperature of a magnetic layer on a disk substrate depends upon the power of laser beams irradiated on the disk and an actual temperature distribution on the substrate differs for individual drives, individual disks or environmental conditions such as temperature depending upon the aberration of lenses in the optical head, the slant of a light beam, a servo accuracy or the like, even if the setting of a laser power is accurate.
Furthermore, since reproduction conditions principally depend upon the temperature distribution on a disk, as mentioned above, jitter characteristics more greatly depend upon the reproduction compared to a record reproduction apparatus of ordinary magneto-optic schemes. FIG. 7 shows a relation between the reproduction power and the jitter quantity. The jitter of a reproduction signal is found to largely vary with different reproduction Power. To implement a high recording density under tolerances of the aberration of an optical system, the accuracy of individual parts, the control error of power and the like in the case of performing magnetic-wall displacement like this, there is a necessity for accurately setting the laser power at the time of reproduction.
In view of the above former problems, it is an object of the present invention to provide a reproduction power test method and an optical information reproduction apparatus capable of accurately setting the optimal reproduction power independently of the aberration of an optical system, the accuracy of individual parts and further the control error of power or the like.
And, the above object is achieved by a reproduction power test method comprising the steps of detecting the domain wall displacement start reproduction power Prdwd at which domain wall starts displacement and the maximum reproduction power Prmax allowing the domain wall displacement; and setting an optimal reproduction power Pr on the basis of the obtained domain wall displacement start reproduction power Prdwd and the maximum reproduction power Prmax.
Besides, the object is achieved also by an optical information reproduction apparatus executing the above reproduction power test method prior to the reproduction of an information.
Further details will be elucidated in embodiments mentioned later.